Diet of a dying star

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AUSTIN, Texas — Scientists are beginning to sort out the stellar ingredients that produce a type 1a supernova, a type of cosmic explosion that has been used to measure the universe’s accelerating expansion.

Two teams of researchers presented new data about these supernovas at the American Astronomical Society meeting on January 11. One team confirmed a long-held suspicion about the kind of star that explodes, and the second provided new evidence for what feeds that star until it bursts.

“This is a confirmation of a decades-old belief, namely that a type 1a supernova comes from the explosion of a carbon-oxygen white dwarf,” said Joshua Bloom, an astronomer at the University of California, Berkeley.

Bloom and his colleagues have been studying supernova 2011fe, the explosion that became visible 21 million light-years away, near the Pinwheel Galaxy, in August. When the PIRATE telescope in Majorca, Spain, wasn’t able to detect the supernova just hours after it exploded, Bloom’s team could set better limits on the size of the star that exploded. They concluded it must have been a white dwarf. When the dwarf — fed by a companion star — gets too heavy, a runaway thermonuclear reaction ignites in its core, producing a fireball bright enough to outshine surrounding galaxies.

But the culprit behind the dwarf’s mass gain is still a mystery: Although scientists know a companion star is feeding the dwarf, they don’t know what type of star that companion is.

Now, astronomers from Louisiana State University in Baton Rouge have answered that question for a centuries-old explosion. The team focused on a bubble-shaped remnant — the remains of a type 1a explosion that occurred 400 years ago — in the nearby galaxy the Large Magellanic Cloud. The remnant, called SNR0509-67.5, now spans 23 light years.

“It’s a beautifully symmetric remnant,” says graduate student Ashley Pagnotta, a coauthor on the team’s paper, which appears in the January 12 Nature. “We could find the center very precisely.”

The bubble’s center is the likely site of the explosion, and, since a large companion star would have survived the explosion and been flung outward at a predictable speed, the team calculated how far from that point a companion might have traveled over the last 400 years.

But they saw no stars within that region, suggesting that the star responsible for inflating the dwarf to explosive proportions was also destroyed. That result pointed to a second white dwarf as the companion, which instead of being chucked from the epicenter would have been shredded and destroyed.

“That’s not what we’d expected,” Pagnotta says. “This is the first supernova for which we’ve been able to make a definitive claim like that.”

Scientists have differing theories about what kind of star feeds a white dwarf. Some, like Pagnotta, suggest a second white dwarf; others think the companion must be a larger, main-sequence star like the sun — or bigger. Different starting ingredients might produce supernovas with different light curves and spectra — the output that lets scientists measure cosmological distances and calculate the rate of the universe’s expansion.

Understanding type 1a “progenitor” systems is crucial for refining these measurements and seeing how the resulting explosions differ, says astronomer Peter Nugent of Lawrence Berkeley National Laboratory in California.